Iron group

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In chemistry and physics, the iron group refers to elements that are in some way related to iron; mostly in period (row) 4 of the periodic table. The term has different meanings in different contexts.

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In chemistry, the term is largely obsolete, but it often means iron, cobalt, and nickel, also called the iron triad; [1] or, sometimes, other elements that resemble iron in some chemical aspects.

In astrophysics and nuclear physics, the term is still quite common, and it typically means those three plus chromium and manganese—five elements that are exceptionally abundant, both on Earth and elsewhere in the universe, compared to their neighbors in the periodic table. Titanium and vanadium are also produced in Type Ia supernovae. [2]

General chemistry

In chemistry, "iron group" used to refer to iron and the next two elements in the periodic table, namely cobalt and nickel. These three comprised the "iron triad". [1] They are the top elements of groups 8, 9, and 10 of the periodic table; or the top row of "group VIII" in the old (pre-1990) IUPAC system, or of "group VIIIB" in the CAS system. [3] These three metals (and the three of the platinum group, immediately below them) were set aside from the other elements because they have obvious similarities in their chemistry, but are not obviously related to any of the other groups. The iron group and its alloys exhibit ferromagnetism.

The similarities in chemistry were noted as one of Döbereiner's triads and by Adolph Strecker in 1859. [4] Indeed, Newlands' "octaves" (1865) were harshly criticized for separating iron from cobalt and nickel. [5] Mendeleev stressed that groups of "chemically analogous elements" could have similar atomic weights as well as atomic weights which increase by equal increments, both in his original 1869 paper [6] and his 1889 Faraday Lecture. [7]

Analytical chemistry

In the traditional methods of qualitative inorganic analysis, the iron group consists of those cations which

The main cations in the iron group are iron itself (Fe2+ and Fe3+), aluminium (Al3+) and chromium (Cr3+). [8] If manganese is present in the sample, a small amount of hydrated manganese dioxide is often precipitated with the iron group hydroxides. [8] Less common cations which are precipitated with the iron group include beryllium, titanium, zirconium, vanadium, uranium, thorium and cerium. [9]

Astrophysics

The iron group in astrophysics is the group of elements from chromium to nickel, which are substantially more abundant in the universe than those that come after them – or immediately before them – in order of atomic number. [10] The study of the abundances of iron group elements relative to other elements in stars and supernovae allows the refinement of models of stellar evolution.

Abundances of the chemical elements in the Solar System. The scale of the vertical axis is logarithmic. Hydrogen and helium are most common, from the Big Bang. The next three elements (Li, Be, B) are rare because they are poorly synthesized in the Big Bang and also in stars. The two general trends in the remaining stellar-produced elements are: (1) an alternation of abundance in elements as they have even or odd atomic numbers, and (2) a general decrease in abundance, as elements become heavier. The "iron peak" may be seen in the elements near iron as a secondary effect, increasing relative abundances of elements with nuclei most strongly bound. SolarSystemAbundances.png
Abundances of the chemical elements in the Solar System. The scale of the vertical axis is logarithmic. Hydrogen and helium are most common, from the Big Bang. The next three elements (Li, Be, B) are rare because they are poorly synthesized in the Big Bang and also in stars. The two general trends in the remaining stellar-produced elements are: (1) an alternation of abundance in elements as they have even or odd atomic numbers, and (2) a general decrease in abundance, as elements become heavier. The "iron peak" may be seen in the elements near iron as a secondary effect, increasing relative abundances of elements with nuclei most strongly bound.

The explanation for this relative abundance can be found in the process of nucleosynthesis in certain stars, specifically those of about 8–11  Solar masses. At the end of their lives, once other fuels have been exhausted, such stars can enter a brief phase of "silicon burning". [11] This involves the sequential addition of helium nuclei 4
2He
 (an "alpha process") to the heavier elements present in the star, starting from 28
14Si
 :

28
14Si
 		+ 4
2He
 		→  32
16S
32
16S
 		+ 4
2He
 		→  36
18Ar
36
18Ar
 		+ 4
2He
 		→  40
20Ca
40
20Ca
 		+ 4
2He
 		→  44
22Ti
   [note 1]
44
22Ti
 		+ 4
2He
 		→  48
24Cr
48
24Cr
 		+ 4
2He
 		→  52
26Fe
52
26Fe
 		+ 4
2He
 		→  56
28Ni

All of these nuclear reactions are exothermic: the energy that is released partially offsets the gravitational contraction of the star. However, the series ends at 56
28Ni
, as the next reaction in the series

56
28Ni
 		+ 4
2He
 		→  60
30Zn

is endothermic. With no further source of energy to support itself, the core of the star collapses on itself while the outer regions are blown off in a Type II supernova. [11]

Nickel-56 is unstable with respect to beta decay, and the final stable product of silicon burning is 56
26Fe
 .

56
28Ni
 		→  56
27Co
  		+  β+   t1/2 = 6.075(10) d
56
27Co
 		→ 56
26Fe
 		+ β+  t1/2 = 77.233(27) d
 Nuclide mass [12] Mass defect [13] Binding energy
per nucleon [14]
62
28Ni
61.9283451(6) u0.5700031(6) u8.563872(10) MeV
58
26Fe
57.9332756(8) u0.5331899(8) u8.563158(12) MeV
56
26Fe
55.9349375(7) u0.5141981(7) u8.553080(12) MeV

It is often incorrectly stated that iron-56 is exceptionally common because it is the most stable of all the nuclides. [10] This is not quite true: 62
28Ni
 and 58
26Fe
 have slightly higher binding energies per nucleon – that is, they are slightly more stable as nuclides – as can be seen from the table on the right. [15] However, there are no rapid nucleosynthetic routes to these nuclides.

In fact, there are several stable nuclides of elements from chromium to nickel around the top of the stability curve, accounting for their relative abundance in the universe. The nuclides which are not on the direct alpha-process pathway are formed by the s-process, the capture of slow neutrons within the star.

The curve of binding energy per nucleon (calculated from the nuclear mass defect) against the number of nucleons in the nucleus. Iron-56 is labelled near the very top of the curve: it can be seen that the "peak" is quite flat, which explains the existence of several common elements around iron. Binding energy curve - common isotopes.svg
The curve of binding energy per nucleon (calculated from the nuclear mass defect) against the number of nucleons in the nucleus. Iron-56 is labelled near the very top of the curve: it can be seen that the "peak" is quite flat, which explains the existence of several common elements around iron.

See also

Notes and references

Notes

  1. ↑ In lighter stars, with less gravitational pressure, the alpha process is much slower and effectively stops at this stage as titanium-44 is unstable with respect to beta decay (t1/2 = 60.0(11) years).

Related Research Articles

Relative atomic mass, also known by the deprecated synonym atomic weight, is a dimensionless physical quantity defined as the ratio of the average mass of atoms of a chemical element in a given sample to the atomic mass constant. The atomic mass constant is defined as being 1/12 of the mass of a carbon-12 atom. Since both quantities in the ratio are masses, the resulting value is dimensionless. These definitions remain valid even after the 2019 redefinition of the SI base units.

The abundance of the chemical elements is a measure of the occurrence of the chemical elements relative to all other elements in a given environment. Abundance is measured in one of three ways: by mass fraction, by mole fraction, or by volume fraction. Volume fraction is a common abundance measure in mixed gases such as planetary atmospheres, and is similar in value to molecular mole fraction for gas mixtures at relatively low densities and pressures, and ideal gas mixtures. Most abundance values in this article are given as mass fractions.

Actinium (89Ac) has no stable isotopes and no characteristic terrestrial isotopic composition, thus a standard atomic weight cannot be given. There are 33 known isotopes, from 204Ac to 236Ac, and 7 isomers. Three isotopes are found in nature, 225Ac, 227Ac and 228Ac, as intermediate decay products of, respectively, 237Np, 235U, and 232Th. 228Ac and 225Ac are extremely rare, so almost all natural actinium is 227Ac.

Naturally occurring cerium (58Ce) is composed of 4 stable isotopes: 136Ce, 138Ce, 140Ce, and 142Ce, with 140Ce being the most abundant and the only one theoretically stable; 136Ce, 138Ce, and 142Ce are predicted to undergo double beta decay but this process has never been observed. There are 35 radioisotopes that have been characterized, with the most stable being 144Ce, with a half-life of 284.893 days; 139Ce, with a half-life of 137.640 days and 141Ce, with a half-life of 32.501 days. All of the remaining radioactive isotopes have half-lives that are less than 4 days and the majority of these have half-lives that are less than 10 minutes. This element also has 10 meta states.

Indium (49In) consists of two primordial nuclides, with the most common (~ 95.7%) nuclide (115In) being measurably though weakly radioactive. Its spin-forbidden decay has a half-life of 4.41×1014 years.

Naturally occurring niobium (41Nb) is composed of one stable isotope (93Nb). The most stable radioisotope is 92Nb with a half-life of 34.7 million years. The next longest-lived niobium isotopes are 94Nb and 91Nb with a half-life of 680 years. There is also a meta state of 93Nb at 31 keV whose half-life is 16.13 years. Twenty-seven other radioisotopes have been characterized. Most of these have half-lives that are less than two hours, except 95Nb, 96Nb and 90Nb. The primary decay mode before stable 93Nb is electron capture and the primary mode after is beta emission with some neutron emission occurring in 104–110Nb.

Germanium (32Ge) has five naturally occurring isotopes, 70Ge, 72Ge, 73Ge, 74Ge, and 76Ge. Of these, 76Ge is very slightly radioactive, decaying by double beta decay with a half-life of 1.78 × 1021 years (130 billion times the age of the universe).

Naturally occurring zinc (30Zn) is composed of the 5 stable isotopes 64Zn, 66Zn, 67Zn, 68Zn, and 70Zn with 64Zn being the most abundant. Twenty-five radioisotopes have been characterised with the most abundant and stable being 65Zn with a half-life of 244.26 days, and 72Zn with a half-life of 46.5 hours. All of the remaining radioactive isotopes have half-lives that are less than 14 hours and the majority of these have half-lives that are less than 1 second. This element also has 10 meta states.

Naturally occurring nickel (28Ni) is composed of five stable isotopes; 58
Ni
, 60
Ni
, 61
Ni
, 62
Ni
 and 64
Ni
, with 58
Ni
 being the most abundant. 26 radioisotopes have been characterised with the most stable being 59
Ni
 with a half-life of 76,000 years, 63
Ni
 with a half-life of 100.1 years, and 56
Ni
 with a half-life of 6.077 days. All of the remaining radioactive isotopes have half-lives that are less than 60 hours and the majority of these have half-lives that are less than 30 seconds. This element also has 8 meta states.

Naturally occurring cobalt (27Co) consists of a single stable isotope, 59Co. Twenty-eight radioisotopes have been characterized; the most stable are 60Co with a half-life of 5.2714 years, 57Co, 56Co, and 58Co. All other isotopes have half-lives of less than 18 hours and most of these have half-lives of less than 1 second. This element also has 11 meta states, all of which have half-lives of less than 15 minutes.

Naturally occurring manganese (25Mn) is composed of one stable isotope, 55Mn. 26 radioisotopes have been characterized, with the most stable being 53Mn with a half-life of 3.7 million years, 54Mn with a half-life of 312.3 days, and 52Mn with a half-life of 5.591 days. All of the remaining radioactive isotopes have half-lives that are less than 3 hours and the majority of these have half-lives that are less than a minute. This element also has 3 meta states.

Naturally occurring iron (26Fe) consists of four stable isotopes: 5.845% of 54Fe (possibly radioactive with a half-life over 4.4×1020 years), 91.754% of 56Fe, 2.119% of 57Fe and 0.286% of 58Fe. There are 24 known radioactive isotopes, the most stable of which are 60Fe (half-life 2.6 million years) and 55Fe (half-life 2.7 years).

Naturally occurring chromium (24Cr) is composed of four stable isotopes; 50Cr, 52Cr, 53Cr, and 54Cr with 52Cr being the most abundant (83.789% natural abundance). 50Cr is suspected of decaying by β+β+ to 50Ti with a half-life of (more than) 1.8×1017 years. Twenty-two radioisotopes, all of which are entirely synthetic, have been characterized, the most stable being 51Cr with a half-life of 27.7 days. All of the remaining radioactive isotopes have half-lives that are less than 24 hours and the majority of these have half-lives that are less than 1 minute. This element also has two meta states, 45mCr, the more stable one, and 59mCr, the least stable isotope or isomer.

Naturally occurring vanadium (23V) is composed of one stable isotope 51V and one radioactive isotope 50V with a half-life of 2.71×1017 years. 24 artificial radioisotopes have been characterized (in the range of mass number between 40 and 65) with the most stable being 49V with a half-life of 330 days, and 48V with a half-life of 15.9735 days. All of the remaining radioactive isotopes have half-lives shorter than an hour, the majority of them below 10 seconds, the least stable being 42V with a half-life shorter than 55 nanoseconds, with all of the isotopes lighter than it, and none of the heavier, have unknown half-lives. In 4 isotopes, metastable excited states were found (including 2 metastable states for 60V), which adds up to 5 meta states.

Naturally occurring titanium (22Ti) is composed of five stable isotopes; 46Ti, 47Ti, 48Ti, 49Ti and 50Ti with 48Ti being the most abundant. Twenty-one radioisotopes have been characterized, with the most stable being 44Ti with a half-life of 60 years, 45Ti with a half-life of 184.8 minutes, 51Ti with a half-life of 5.76 minutes, and 52Ti with a half-life of 1.7 minutes. All of the remaining radioactive isotopes have half-lives that are less than 33 seconds, and the majority of these have half-lives that are less than half a second.

<span class="mw-page-title-main">Iron–nickel alloy</span>

An iron–nickel alloy or nickel–iron alloy, abbreviated FeNi or NiFe, is a group of alloys consisting primarily of the elements nickel (Ni) and iron (Fe). It is the main constituent of the "iron" planetary cores and iron meteorites. In chemistry, the acronym NiFe refers to an iron–nickel catalyst or component involved in various chemical reactions, or the reactions themselves; in geology, it refers to the main constituents of telluric planetary cores.

<span class="mw-page-title-main">Standard atomic weight</span> Relative atomic mass as defined by IUPAC (CIAAW)

The standard atomic weight of a chemical element (symbol Ar°(E) for element "E") is the weighted arithmetic mean of the relative isotopic masses of all isotopes of that element weighted by each isotope's abundance on Earth. For example, isotope 63Cu (Ar = 62.929) constitutes 69% of the copper on Earth, the rest being 65Cu (Ar = 64.927), so

Nickel-62 is an isotope of nickel having 28 protons and 34 neutrons.

Aaldert Hendrik Wapstra was a Dutch physicist.

<span class="mw-page-title-main">Isotope</span> Different atoms of the same element

Isotopes are distinct nuclear species of the same element. They have the same atomic number and position in the periodic table, but differ in nucleon numbers due to different numbers of neutrons in their nuclei. While all isotopes of a given element have almost the same chemical properties, they have different atomic masses and physical properties.

References

  1. 1 2 M. Green, ed. (2002): Organometallic Chemistry , volume 10, page 283. Royal Society of Chemistry; 430 pages, ISBN   9780854043330
  2. ↑ Bravo, E. (2013). "Insights into thermonuclear supernovae from the incomplete Si-burning process". Astronomy & Astrophysics. 550: A24. arXiv: 1212.2410 . Bibcode:2013A&A...550A..24B. doi:10.1051/0004-6361/201220309. S2CID   49331289.
  3. ↑ Sherwood Taylor, F. (1942), Inorganic and Theoretical Chemistry (6th ed.), London: Heinemann, pp. 151–54, 727–28.
  4. ↑ Strecker, A. (1859), Theorien und Experimente zur Bestimmung der Atomgewichte der Elemente, Braunschweig: Friedrich Vieweg.
  5. ↑ "Proceedings of Societies [Report on the Law of Octaves]", Chemical News, 13: 113, 1866.
  6. ↑ Mendelejeff, D. (1869), "On the Relationship of the Properties of the Elements to their Atomic Weights", Z. Chem., 12: 405–6.
  7. ↑ Mendeléeff, D. (1889), "The Periodic Law of the Chemical Elements", J. Chem. Soc., 55: 634–56, doi:10.1039/ct8895500634 .
  8. 1 2 Vogel, Arthur I. (1954), A Textbook of Macro and Semimicro Qualitative Inorganic Analysis (4th ed.), London: Longman, pp. 260–78, ISBN   0-582-44367-9 .
  9. ↑ Vogel, Arthur I. (1954), A Textbook of Macro and Semimicro Qualitative Inorganic Analysis (4th ed.), London: Longman, pp. 592–611, ISBN   0-582-44367-9 .
  10. 1 2 Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 13–16. ISBN   978-0-08-022057-4..
  11. 1 2 Woosley, Stan; Janka, Thomas (2005), "The Physics of Core-Collapse Supernovae", Nature Physics, 1 (3): 147–54, arXiv: astro-ph/0601261 , Bibcode:2005NatPh...1..147W, CiteSeerX   10.1.1.336.2176 , doi:10.1038/nphys172, S2CID   118974639 .
  12. ↑ Wapstra, A.H.; Audi, G.; Thibault, C. (2003), The AME2003 Atomic Mass Evaluation (Online ed.), National Nuclear Data Center . Based on:
  13. ↑ Particle Data Group (2008), "Review of Particle Physics" (PDF), Phys. Lett. B, 667 (1–5): 1–6, Bibcode:2008PhLB..667....1A, doi:10.1016/j.physletb.2008.07.018, hdl: 1854/LU-685594 , S2CID   227119789, archived from the original (PDF) on 2020-09-07, retrieved 2019-12-13. Data tables.
  14. ↑ Mohr, Peter J.; Taylor, Barry N.; Newell, David B. (2008). "CODATA Recommended Values of the Fundamental Physical Constants: 2006" (PDF). Reviews of Modern Physics . 80 (2): 633–730. arXiv: 0801.0028 . Bibcode:2008RvMP...80..633M. doi:10.1103/RevModPhys.80.633. Archived from the original (PDF) on 2017-10-01. Direct link to value .
  15. ↑ Fewell, M. P. (1995), "The atomic nuclide with the highest mean binding energy", Am. J. Phys., 63 (7): 653–58, Bibcode:1995AmJPh..63..653F, doi:10.1119/1.17828 .
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